Astronomy changed forever 50 years ago this week, when scientists first began to understand the true nature of quasars, the brightest objects in the universe.

On March 16, 1963, Dutch-born astronomer Maarten Schmidt published the first-ever definitive measurement of the distance to a quasar, finding that a puzzling object called 3C 273 lies about 2.5 billion light-years from Earth.

The stunning discovery showed that 3C 273's inherent luminosity must be off the charts for it to appear so bright despite its immense distance. The object was no star, despite the starlike characteristics of its light emissions. ("Quasar" is short for "quasi-stellar radio source.")

Over time, astronomers figured out that quasars actually shine from the cores of galaxies, blazing forth when the supermassive black holes that lurk there gobble up huge quantities of gas, dust and other matter. Still, there remain many mysteries about quasars that researchers have yet to solve.

SPACE.com caught up with Schmidt, now a professor emeritus at Caltech in Pasadena, this week to discuss quasars, how they have improved our understanding of the universe and how he feels on the cusp of this big anniversary:

SPACE.com: So how does it feel now, 50 years after making that seminal discovery?

Schmidt: Oh, clearly, yes. I certainly saw in the spectrum of 3C 273 that it was quite simply the hydrogen spectrum in emission, with Balmer lines, and that it was shifted by about six of their wavelengths to the red [indicating rapid motion away from the observer; more distant objects are receding at a greater rate]. And since the object looked like a star, this was stunning. What was happening?

It clearly was not a star; stars don't do that. They have very small redshifts — up to 0.2 percent because of their movements, never bigger. And this was 16 percent, 80 times larger. So what I was looking at could not be an ordinary star at all.

Interpreting it as a cosmological redshift, which I soon did because it was so bright in the sky — the luminosity turned out to be very high. And that was remarkable, because it was immensely brighter than normal galaxies, even the biggest galaxies.

So here you have something that is out in the universe, it's more luminous than an entire galaxy, and it looks like a star. It was an astounding experience.

SPACE.com: Where does that moment rank in your career?

Schmidt: It was the most important finding in my whole career. Most other work that I did was long work, first in Holland, on 21-centimeter [emission wavelength] work with galactic structure, uncovering the spiral structure in our galaxy with 21 centimeters in the middle '50s, and then later on certain other work. But this beat it all.

And the funny thing is, that while most of those other studies were long studies of years and years, this all came out more or less in a month — technically, in an afternoon. [Most Powerful Quasar Discovered (Video)]

SPACE.com: How has discovering the nature of quasars impacted astronomy and our understanding of the universe?

Schmidt: There are two aspects to it. The astrophysics was really important, because they were black holes, and apparently they were accreting material. After all, a black hole is black if you don't make it accrete stuff from the neighborhood. But if you feed it stuff, you get a very luminous accretion disk that is in fact much brighter than the whole galaxy — hard to believe — and it was that thing that we were seeing. That had not been realized or seen before, and that was very important.

The other thing was that when you looked at the statistics, it turned out that 10 billlion years ago, there were 100 times more of these quasars than now. And that came from [Schmidt's] laborious work on statistics and finding complete samples that are well-defined in terms of magnitude and redshifts and all sorts of other things.

We still have a few [quasars] in the universe, but it looks like soon they will die out. That's rather strange, but of course in a field like this, you cannot limit yourself to the things that you were thinking of that you might be seeing. You have to take it the way it comes.

And they were very difficult to study, because what do you do with a point source? The only thing that you sort of can do is spread out the spectrum and look at the spectrum, and that was done in getting the redshift, and then the conditions of the gas that was emitting — that could all be studied.

But it didn't allow all that much detailed information to be gathered, and so it took also theoretical work eventually — in particular from [Donald] Lynden-Bell at Cambridge in 1969 — to come up with the idea of black holes being involved and accreting material from their environment. [Images: Black Holes of the Universe]

SPACE.com: And quasars also let astronomers peer far back in time, don't they?

Schmidt: Yes. Since quasars are so bright, you can see them at very large distances. And from these large distances, it takes the light many years to get here.

So necessarily with the quasars, you look back in time. You look back at objects as they were long ago — the larger the redshift, the longer ago. So that is an important aspect of the whole study — that you can document the evolution of the universe and its contents, starting with quasars.

These days, after 50 years, it can also be done with other objects like gamma-ray bursts and even galaxies. But with quasars, because they were so bright and luminous, it could be done immediately after I discovered the redshift.

So already in 1965 I think, and certainly in 1966, I had already developed in the scientific literature quite a complete idea and results of how quasars evolved in the universe, from the beginning until today. And that work to quite a degree still stands.

SPACE.com: How do you feel about the current state of the field? Have we made a lot of progress toward understanding quasars in the past half-century?

Schmidt: The understanding has not developed very much in those 50 years. Tomorrow [Thursday] in [the journal] Nature, you'll find a comment from an astronomer who is almost criticizing astronomy for not achieving this.

But it's a difficult problem. And as I said, one part of why it is difficult is that you only see a point source. So you don't see its structure; it's a difficult thing to get ahold of. You can't help it.

SPACE.com: How can we gain a better understanding of what drives quasars? Do we need better instruments? Do we need better theory?

Schmidt: We have to go to the biggest possible resolution in space, in sort of angle, so that you can try to make a map of the quasar.

Now that is not impossible, because in very large baseline radio astronomy, where you have interferometers that span over very many miles — sometimes even across the Earth — with a large number of radio antennas all working on the same quasar at the same time, you can come up with a map that has exquisite resolution in angle. And what you then see is very interesting, because you see that there is motion outward.

And originally, in the middle '70s, it was claimed that this motion was faster than the velocity of light. But it turned out later on that this had to do with angles and so on, and there is in reality no such effect. But these motions out of the centers of quasars happen themselves with a speed which is a large fraction of that of light.

So that is the beginning — that was the beginning; it's already been many years — of fascinating studies about, on the very small scale, the development of quasars, their structure, and how, believe it or not, from month to month, that structure changes.

SPACE.com: What's the biggest quasar mystery yet to be solved?

Schmidt: That's hard to say. I think, in detail, understanding the source of the energy is probably what is still lacking. But when I say "in detail," I must admit a certain ignorance, and I couldn't explain what I have in mind further. But that's my suspicion, that that's still somewhat weak.

SPACE.com: Are you doing anything to celebrate the 50-year anniversary?

Schmidt: Well, at Caltech there will be an international symposium, which will be held on September 9 and 10. So that will be the occasion.

SPACE.com: But you're not doing anything on a personal level?

Schmidt: No, nothing in particular. I'll find myself tonight [Wednesday] talking to the BBC, perhaps live, but that's the end of the publicity this week.